1. Field of the Invention
The invention relates to a process for manufacturing a strip of steel sheet for the production of metal packaging and the sheet obtained by the process.
2. Discussion of Background
In order to manufacture steel packaging products by drawing, such as cans for food products or for drinks, blanks cut from thin sheets are used. These sheets must have good formability to that they can be drawn and they must also have good mechanical strength depending on their end use.
It is necessary to obtain sheets having good mechanical properties, in particular a high mechanical strength and a high yield stress, in order to produce bottoms or bodies of very-high-performance metal cans, and also in the case of very thin sheets for the production of certain types of packaging.
In particular, double-reduction (DR)-type sheets whose mechanical strength is greater than or equal to 550 MPa are obtained from hot sheets, by successively carrying out a first cold-rolling operation, an operation of annealing, generally continuous annealing, of the cold-rolled sheet and then a second cold-rolling operation comprising two successive passes generally carried out on a skin-pass mill.
However, the high mechanical properties of the sheets are obtained to the detriment of the formability of these sheets after the second pass of the final cold rolling. It is desirable to remedy this drawback.
In particular, it is desirable to obtain sheets which have high mechanical properties and good drawability at necking.
The level of mechanical properties of the sheet, which is obtained after the second cold rolling, depends on the reduction ratio or draw ratio of the sheet obtained during the two passes of the second rolling. Of course, in the case of high draw ratios, the high work hardening of the sheets is accompanied by a poor formability.
The double-reduction process, which involves a second cold-rolling operation in two passes after the sheets have been annealed, has been applied to grades of packaging steel, both of the ultra-low-carbon (ULC) type, the carbon content of which is less than 0.008%, and to steels of other types, for example low-renitrided-aluminium steels containing from 8 to 16 thousandths of a per cent of nitrogen. In all cases, obtaining higher mechanical properties means a decrease in the formability.